In this lab, you’ll connect a digital input circuit and a digital output circuit to a microcontroller. Though this is written for the Arduino microcontroller module, the principles apply to any microcontroller.

Digital input and output are the most fundamental physical connections for any microcontroller. The pins to which you connect the circuits shown here are called General Purpose Input-Output, or GPIO, pins. Even if a given project doesn’t use digital in and out, you’ll often use LEDs and switches during the development for testing whether everything’s working.

What You’ll Need to Know

To get the most out of this lab, you should be familiar with the following concepts and you should install the Arduino IDE on your computer. You can check how to do so in the links below:

Note on Pulldown Resistor

What happens if you don’t include the resistor connecting the switch to ground?

The resistor connecting the switch is a pulldown resistor. It provides the digital input pin with a reference to ground. Without it, the input will behave unreliably.

If you don’t have a 10-kilohm resistor for the switch, you can use any reasonably high value. 4.7K, 22K, and even 1 Megohm resistors have all been tested with this circuit and they work fine. See the digital input and output notes for more about the digital input circuit.

Schematic view

Arduino with switch on digital pin 2

Add Digital Outputs (LEDs)

Connect a 220-ohm resistor and an LED in series to digital pin 3 and another to digital pin 4 of the Arduino.

Schematic view

Add LEDs on pins 3 and 4

Note on LED Resistor Values

For the resistor on the LED, the higher the resistor value, the dimmer your LED will be. So 220-ohm resistors give you a nice bright LED, 1-kilohm will make it dimmer, and 10K or higher will likely make it too dim to see. Related video: Resistors for an LED

Program the Arduino

Connect the microcontroller to your computer via USB. When you plug the Arduino into your computer, you’ll find a new serial port in the Tools–>Serial Port menu (for details on installing the software, and USB-to-serial drivers for older Arduino models, see the Arduino Getting Started Guide). In OSX, the name will begin with /dev/tty.usbmodem- if it’s an Arduino Uno later, and /dev/tty.usbserial- if it’s a Duemilanove or earlier. In Windows it will start with COM like all the other serial ports.

Now it’s time to write a program that reads the digital input on pin 2. When the switch is pressed, turn the yellow LED on and the red one off. When the switch is released, turn the red LED on and the yellow LED off.

In your setup() method, you need to set the three pins you’re using as inputs or outputs, appropriately.

void setup() {
pinMode(2, INPUT); // set the switch pin to be an input
pinMode(3, OUTPUT); // set the yellow LED pin to be an output
pinMode(4, OUTPUT); // set the red LED pin to be an output
}

In the main loop, first you need an if-then-else statement to read the switch.

void loop() {
// read the switch input:
if (digitalRead(2) == HIGH) {
// if the switch is closed:
digitalWrite(3, HIGH); // turn on the yellow LED
digitalWrite(4, LOW); // turn off the red LED
}
else {
// if the switch is open:
digitalWrite(3, LOW); // turn off the yellow LED
digitalWrite(4, HIGH); // turn on the red LED
}
}

Once you’re done with that, you’re ready to compile your sketch and upload it. Click the Verify button to compile your code. Then click the Upload button to upload the program to the module. After a few seconds, the following message will appear in the message pane to tell you the program was uploaded successfully. Related video: Upload the code to the Arduino

Binary sketch size: 5522 bytes (of a 7168 byte maximum)

Press the switch and watch the LEDs change until you get bored. That’s all there is to basic digital input and output!

Get Creative

Many projects can be made with just digital input and output. For example, a combination lock is just a series of switches that have been switched in a particular sequence. Consider the cymbal-playing monkey below:

His cymbals can be turned into a switch by lining them with tin foil and screwing wires to them:

Those wires can be run to a breadboard and used as a switch. Then the microcontroller could be programmed to listen for pattern of cymbal crashes, and if it sees that pattern, to open a lock by switching on a digital output.

Now come up with your own physical interface for a combination lock, for example.